Anti sound

You may be wondering how my idea of Active Walls might work and what is taking place.

Imagine taking one of your loudspeakers and placing it against a wall, facing into the room. Next to it place a microphone pointed in the same direction. If the microphone is connected to a clever circuit, the sound picked up can be rebroadcast by the speaker out of phase, actively canceling sound. Once cancelled, the sound that would formerly have reflected off the wall and returned to your ears is cancelled. You have created a non-reflective wall.

Active noise cancelation is nothing new. Bose and others have used it for years in their noise canceling headphones. The headphone is a small powered loudspeaker and outside of the ear covers are tiny microphones that pickup sound, reverse its phase, and rebroadcast it to your ear. Thus, any sound entering the headphones is nulled by the out of phase loudspeaker output. Here are two diagrams I found on the internet describing how this works:

In the destructive interference picture you can see as one wave goes up, the other opposing wave goes down, creating a sum of zero. Think of a vessel with water and a pump. If the pump is pulling water out of the vessel it drains quickly. If another pump pushes water in, the vessel fills. If both pumps push and pull at the same time and in equal measure, the water level never moves. The push of one pump cancels the pull of the second pump.

That is how active noise canceling works. One of the challenges of mechanizing such a scheme with loudspeakers is the microphone will also hear that which the speaker produces as well as what’s around it, a problem not encountered when using headphones.

About Paul McGowan

Paul McGowan is the co-founder of PS Audio (The 'P' ) and has been designing, building and enjoying high end audio since 1974. He lives in Boulder Colorado with his wife Terri and his four sons: Lon, Sean, Scott and Rob. His hobbies include hiking, skiing, cooking, artisan bread baking. His current big project, other than playing with stereos, is writing a book series called the Carbon Chronicles. Book One, the Lost Chronicle, is a work in progress. You can view his efforts at http://www.paulmcgowan.com

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12 COMMENTS

How does this concept differ, Paul, from digital room correction systems (DRCs) offered by companies like TacT, Lyngdorf, Trinnov and others, concepts that additionally claim to correct a non-horizontal frequency curve of the speakers? My personal experience with these concepts is that you still have to find the optimal listening place having the minimal room modes because of the impossibility of totally correcting room mode effects. The listening effect reached by DRCs is poor compared to the crosstalk effect by RACE algorithms offered by Ralph Glasgal. I fear my expectations getting some practical hints for minimizing the effects of floor and ceiling reflections will not be fulfilled here. Regards

It is obvious, Paul, that I didn’t get your idea here. How can the wizard box separate the sound reflections from the ‘original’ sound? By comparing the signal fed to the speakers with the mic signal? Is your ‘wall problem’ primarily a problem of the typical American drywall construction I am not familiar with?

As Acuvox has pointed out you cannot separate recorded reflections. They are permanently attached. But we don’t want to do that. Just imagine leaving the stereo system and the recording alone – keep it untouched – and I am proposing changing the room size the stereo system is played in. Like moving your stereo system into a much larger room. Think of how different it would sound, regardless of wall construction.

The ability to electronically control the size of the room itself is the basis of my idea.

Ok, Paul, I think I’ve got the basic idea now: simulating a larger room. That idea fit’s to my more pragmatic approach of preferring nearfield listening with point-source speakers in a given normal listening room. As I stated an old experience earlier: the bigger the room the better! 🙂

I’ve read about technology which can miniaturize a speaker to the size of an LED pixel like in an LED display. I’ve thought about this concept before where a type of thin film flat material like wall paper could be manufactured to do an entire room. I think it could be done some day but would be expensive. However, I seriously doubt it would be worth the effort.

Wipe out something you don’t want by colliding it with its exact opposite, kind of like the Bizarro Superman world. Matter, meet anti-matter. Energy, meet anti energy. We don’t want to wipe out everything, just the parts we don’t want.

Selective phase cancellation is a tool that has been around for a long time. It’s the basis of negative feedback. Distortion, meet anti-distortion. Sounds easy? It isn’t. The math is staggering. Used incorrectly it is a disaster creating more distortion than it was intended to solve. It can even turn an amplifier into an oscillator. Used correctly it can reduce distortion to the point where you can barely measure it. Most wannabe amplifier designers don’t know how to use it successfully and they and many audiophiles who follow them condemn it outright as an evil to be avoided at all cost. How wrong they are. The Zenith compatible FM Stereo system adopted by the FCC uses phase cancellation to generate L and R signals from the L + R signal transmitted to all FM receivers and an L – R signal broadcast on the same carrier frequency upshifted for broadcast and down shifted by the multiplex adapter in the receiver. Add the two in phase and out of phase, adjust the amplitude properly and voila, L and R are separated through simple algebraic addition.

Now enter the world of three dimensions where amplitude and time join space. The problem of phase cancellation gets much harder in the world of solid geometry. Now the out of phase energy must not only coincide in amplitude and time but in three dimension of space as well. In the controlled small space of a headphone this is difficult enough. I nearly bought a pair some years ago when I did a lot of travel for work. Of three brands I tried, Bose worked much better than Philips or Sony models IMO. However, it only works well at low and mid frequencies for noise cancellation. As frequency increases and wavelengths get very short, the ability to cancel sound this way by making them coincide in space becomes much harder. Instead the passive seal around the ear blocks out high frequency noise from reaching the ear.

Ambiophonic sound and its like use phase cancellation to null out the sound of the left speaker reaching the right ear and the sound of the right speaker reaching the left ear. It does this by adding the right channels signal to the left channel speaker with precisely the right time delay and amplitude and visa versa for the other channel. To get it to work you must position yourself in precisely the right spot for the cancellation to be effective. It almost seems a miracle that it can be made to work at all but it does. Yet it has the same problem at high frequencies as noise cancellation headphones do.

If it were possible to null out the reflections of a small room and superimpose that on the acoustics of a large room I think you could quit your day job Paul. You could for example build anechoic chambers in an entirely different way. Think of how useful that would be for making noisy areas like open landscape offices quiet. There would be many other applications for it I haven’t thought of. Can it be done? I don’t know how but then again we live in a world of amazing advances that would have seemed impossible to me not so long ago. If you can make this idea work Paul, I think this and not your amplifiers and DACs will be what you’d be remembered for. Anti acoustic energy.

I’m reminded of Henry Kloss’ Sound Space Control, from the late 70s. There was nothing like it at the time. By changing the delay, it attempted to simulate various listening environments, from a cavernous symphony hall to an intimate jazz club. It did a pretty good job of it. It was not noise cancelling, but an effort to change the sound within the space at hand. Mark

I think your idea is hugely exciting on two levels, the first is that even us 0.1% of audio enthusiasts with the extraordinary good fortune to listen yo our systems in truly dedicated listening rooms still have to accept that our rooms interact in a deleterious manner with our speakers, hence the sound we hear in the room. The second problem is that living spaces are getting smaller in many places today, and many of the 20 somethings today will simply have to accept, especially outside of the USA, that their homes will probably be smaller than their parents due to the cost of homes ever escalating in many parts of the World. An audio technology which would allow them to enjoy their systems as though they had a spacious listening room would literally transform the performance envelope which many of these people could enjoy with their systems, it would make high quality audio systems truly viable again for many people who would otherwise simply ignore our marketplace and accept the best they can do are headphones and an ipod / tablet / PC.

I almost think that this idea could be considered on two levels, each with a highly viable and valuable product. The first would be an anti noise ‘box’. If you think about this as a box which could listen to the incoming signal, might perhaps have a way of being able to compensate for the distortions introduced by the cabinet, then anything it hears in the room which is not the signal or the frequency response of the speaker is erradicated via anti-noise, maybe using an extra pair of speakers in the room to generate the cancelling signal. This product would strike me as being a DSP based device, maybe setup with the aid of a PC. It’s not expanding the soundstage electronically, but it is removing the interference from the room. We know this approach will work, not only has it being proven in products such as the Lyngdorf and TACT products, but it’s using technology seen in cars to reduce road noise by feeding anti-noise into the passenger compartment.

The second one will be the ambitious one, using active panels on the wall, but I’m thinking that using the DSP box described above, then using some sort of Distributed Mode loudspeaker panel (first used by MIssion I think in domestic Hi-Fi but designed to feed anti noise into the cab of helicopters) as the wall panel, this would surely give us a starting point.

I know as the CEo you cannot throw money at a pet project recklessly, but this would give you a scaled way of implementing a practical solution which would be useful to enthusiasts aroung the World, and if you used Kickstarter like Scott did with Sprout, then surely the risk exposure for PS Audio could be minimised?

As I said, you got me thinking with this series, so thank you for this, even if it’s nothing more than a thought exercise.

And thanks for reading. It isn’t just money, it’s time and focus. Sure, we could Kickstart something, heck, from a money standpoint we could probably afford to invest in it – but at the expense of what? We currently have about 10 active engineering projects going, from research projects to development projects and limited resources to handle them – so they are carefully scheduled and allocated. We could increase staff but it’s not like one can just go down to the temp agency and hire talented engineers and coders.

Running a business successfully requires focus and focus means shedding those projects that don’t fit the direction of the company as well as others.

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